ARM: imx5x: clean up ARCH_MX5X

Move to SOC_SOC_IMX5X. Leave only places which prevent multi-soc
using ARCH_MX5X.

Signed-off-by: Richard Zhao <richard.zhao@freescale.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>